This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:                3GPP third generation partnership project        BW bandwidth        CB coordinated beamforming        CC component carrier        CDM code division multiplexing        CoMP coordinated multipoint        CQI channel quality indication        CSI channel state information        DL downlink (eNB towards UE)        DM RS demodulation reference signal        eNB E-UTRAN Node B (evolved Node B)        EPC evolved packet core        ePDCCH enhanced physical downlink control channel        E-UTRAN evolved UTRAN (LTE)        HARQ hybrid automatic repeat request        IMT-A international mobile telephony-advanced        ICIC inter-cell interference coordination        ITU international telecommunication union        ITU-R ITU radiocommunication sector        LTE long term evolution of UTRAN (E-UTRAN)        MAC medium access control (layer 2, L2)        MCS Modulation and Coding Scheme        MM/MME mobility management/mobility management entity        Node B base station        OAM operations and maintenance        OFDMA orthogonal frequency division multiple access        PDCCH physical downlink control channel        PDCP packet data convergence protocol        PHY physical (layer 1, L1)        PMI precoding matrix indicator        RB resource block        RLC radio link control        RRC radio resource control        RRM radio resource management        SC-FDMA single carrier, frequency division multiple access        S-GW serving gateway        SRS sounding reference signal        TDD time domain duplex        UE user equipment, such as a mobile station or mobile terminal        UL uplink (UE towards eNB)        UTRAN universal terrestrial radio access network        
One specification of interest is 3GPP TS 36.300, V8.12.0 (2010-04), “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E UTRA) and Evolved Universal Terrestrial Access Network (E UTRAN); Overall description; Stage 2 (Release 8),” incorporated by reference herein in its entirety. This system may be referred to for convenience as LTE Rel-8 (which also contains 3G HSPA and its improvements). In general, the set of specifications given generally as 3GPP TS 36.xyz (e.g., 36.211, 36.311, 36.312, etc.) may be seen as describing the Release 8 (Rel-8) LTE system. More recently, Release 9 (Rel-9) versions of at least some of these specifications have been published including 3GPP TS 36.300, V9.8.0 (2011-10), incorporated by reference herein in its entirety. Even more recently, Release 10 (Rel-10) versions of at least some of these specifications have been published including 3GPP TS 36.300, V10.5.0 (2011-10), incorporated by reference herein in its entirety.
FIG. 1 shows the overall architecture of the E-UTRAN system. The E-UTRAN system includes eNBs 2, 3 and 4, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE (not shown). The eNBs 2, 3 and 4 are interconnected with each other by means of an X2 interface. The eNBs 2, 3 and 4 are also connected by means of an S1 interface to an EPC, more specifically to a MME (Mobility Management Entity) by means of a S1 MME interface and to a Serving Gateway (SGW) by means of a S1 interface. FIG. 1 shows a first MME/S-GW 5 and a second MME-S-GW 6. It should be appreciated that the MME may be a separate entity from the S-GW. The S1 interface supports a many-to-many relationship between MMEs/S-GW and eNBs.
The eNB hosts the following functions:                functions for RRM: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both UL and DL (scheduling);        IP header compression and encryption of the user data stream;        selection of a MME at UE attachment;        routing of User Plane data towards the Serving Gateway;        scheduling and transmission of paging messages (originated from the MME);        scheduling and transmission of broadcast information (originated from the MME or OandM); and        a measurement and measurement reporting configuration for mobility and scheduling.        
Of particular interest herein are the further releases of 3GPP LTE (e.g., LTE Rel-10) targeted towards future IMT-A systems, referred to herein for convenience simply as LTE-Advanced (LTE-A). Reference in this regard may be made to 3GPP TR 36.913, V8.0.1 (2009-03), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Requirements for Further Advancements for E UTRA (LTE-Advanced) (Release 8), incorporated by reference herein in its entirety. A goal of LTE-A is to provide significantly enhanced services by means of higher data rates and lower latency with reduced cost. Reference is further made to a Release 9 version of 3GPP TR 36.913, V9.0.0 (2009-12), incorporated by reference herein in its entirety. Reference is also made to a Release 10 version of 3GPP TR 36.913, V10.0.0 (2011-06), incorporated by reference herein in its entirety.
In downlink (DL) coordinated multi-point (CoMP) design, the achieved gain is typically associated with a high requirement on feedback from a UE (e.g., channel state information (CSI), channel quality indication (CQI), etc.). One option is to provide subband precoding matrix indicator and CQI with a high feedback rate; however, this uses a large uplink (UL) overhead.
The achieved gain may also be associated with frequency/time synchronization among multiple transmission points. One approach to improve gain is to require high frequency/time synchronization requirements. Few existing systems may be able to accommodate these demands. Additionally, systems that can meet the high frequency/time synchronization requirements may be very expensive. Another approach is to limit downlink (DL) CoMP to scenarios where highly accurate frequency/time synchronization can be achieved; however, this severely limits number of cases where DL CoMP can be used.
For CoMP operations, typically tight coupling between eNBs is assumed. For example, baseband pooling and fast broadband fiber connections may exist among multiple eNBs. Also, the cells joined in CoMP transmission may either share the same time/frequency reference bases through a shared local oscillator (as in the case of intra-site CoMP), or highly accurate time/frequency reference bases are made available to the involved cells (as in the case of inter-site CoMP). In addition, for inter-site CoMP, calibration is also assumed for the involved cells. The typical network architecture for CoMP also implies that CoMP is normally enabled on equipments from a single vendor in the deployed network as the required tight coupling is rather difficult to achieve in a multi-vendor deployment environment. It also implies substantial modification to the network connections and network nodes (eNB) transitioning from single-cell operation to CoMP operation. With all the above listed constraints, the applicable scenarios for CoMP operations is severely limited by implementation cost, implied network architecture, vendor-sourcing restriction, network upgrade complications and deployment scenarios.
In CoMP operations, not only the channel information from a UE's home cell, but also that from other cells are needed to make decision on the transmission to one or more UEs. Downlink CQI/CSI feedback and uplink sounding can be used to acquire the needed channel information. As mentioned above, in a network with tightly coupled eNBs, the channel state information can be shared among all eNBs. Thus, scheduling decisions, transmit weight design, power allocation, MCS selection, etc. can be determined jointly or in a coordinated way among the eNBs. Given the substantial CoMP gain available and the associated implementation constraints, it is beneficial to exploit the possibilities with CoMP operations without the associated implementation constraints. If CoMP operations are enabled for eNBs that are loosely coupled, then the applicable scenarios for CoMP operations can be greatly enlarged and many thorny implementation issues can be avoided. Additionally, it will be easier to deploy with equipments from multiple vendors.
The status of using SRS to obtain channel information is reviewed below.                Periodic sounding reference signals (SRS) have been specified in Rel-8/9. UE transmission of SRS can take place at a configured periodicity over configured sub-bands. The SRS transmission can be sub-band or wideband and be with or without hopping. However, this use is intended for a UE's home or serving cell in uplink frequency selective scheduling, downlink frequency scheduling, PMI selection and/or beamforming weight design in a TDD system.        In Rel 10, aperiodic SRS is introduced to solve the multiplexing capability issue and providing more timely channel state information (CSI). In this case, the UE is requested to send the SRS only at the command of the eNB instead of at periodic intervals. This reduces SRS overhead and allows more UEs to share the same SRS resource. In addition, the eNB can request SRS transmission just prior to scheduling the UE for data transmission, thus acquiring more accurate channel state information. This aperiodic SRS can be triggered by an uplink grant or downlink assignment in TDD. However the SRS transmission is still intended for use only in the home cell.        It may be possible that SRS transmissions from a given access point can be detected through a SRS correlator in other access points. This can provide valuable channel state information of a particular UE to the other access points for the purpose of interference avoidance or nulling. However, except in rare cases with extreme UE distribution/traffic type, the downlink transmission in a LTE access point does not persistently go to one UE. Thus, the spatial information captured in other access points cannot be blindly applied for downlink transmission weight design in each of those access points. This is because the eNB cannot assume that the UE whose SRS transmission was detected will indeed be the same UE being scheduled in the downlink.        
If a centralized scheduler or channel state information (CSI) concentrator/distributor exists in the network, the CSI information captured in each access point may be given labels for those access points and their timing. A super-eNB may have a physical limit that put constraints on the network architecture. Consequently, Rel-8/9/10 SRS does not provide a satisfactory solution to Rel-11 CoMP design.
Additional techniques used to attempt to improve CoMP design include increasing SRS capacity and providing SRS for each transmission antenna. Another technique is to use an uplink grant or a downlink assignment to trigger SRS transmissions. However, there remains a need to improve SRS configuration and control design and to provide fresh channel information to multiple access points.
It should be understood in the LTE specifications, SRS and physical random access channel (PRACH) signals share some similarities between them: they are both generated from Zadoff-Chu sequences, and also multiplexing through cyclic shifts and different root sequences are possible. From this, it should be understood that the PRACH signal can also be used as a sounding signal. The description below equally applies to PRACH.
In a first aspect there is provided a method comprising: sending, from a first access point, a scheduling assignment to a first user equipment at a first time; receiving, at the first access point, a reference signal transmission from the first user equipment at a second time; and sending, from the first access point, a downlink data packet transmission to the first user equipment at a third time.
Preferably the method further comprises: estimating, at a second access point at the second time, reference signal resources, over which the reference signal transmission from the first user equipment in the first access point is transmitted, where the reference signal transmission is associated with the downlink data packet transmission from the first access point at the third time; and
choosing a transmission strategy for use at the third time based at least in part on the estimated reference signal resources.
Preferably the downlink data packet transmission at the third time is a coordinated multipoint transmission.
Preferably, where, in coordinated beamforming coordinated multipoint transmission, the received reference signal transmission is used to one of:
design a downlink transmit weight and choose a precoder from a codebook.
Preferably, where, in dynamic access point selection coordinated multipoint transmission, the reference signal transmission is used in coordination with reference signal reception at other access points in a coordinated multipoint transmission set to choose a transmission access point to serve at least one other user equipment at the third time.
Preferably, where, in dynamic blanking coordinated multipoint transmission, the reference signal transmission is used in coordination with reference signal reception at other access points in a coordinated multipoint transmission set to determine whether the second access point should transmit to at least one other user equipment at the third time, where the coordinated multipoint transmission set comprises the second access point.
Preferably the second reference signal transmission is used to design beamforming weights.
Preferably the beamforming weights generate spatial nulling in a direction towards the first user equipment.
Preferably a bandwidth of the reference signal transmission is equal to a bandwidth of a downlink radio barrier.
Preferably the receiving comprises separating the reference signal transmission using a transmission comb.
Preferably reference signal transmissions from user equipment associated with the first access point use a first finger in the transmission comb and reference signal transmissions from user equipment not associated with the first access point use a second finger in the transmission comb.
Preferably reference signal transmissions from user equipment associated with the first access point use a first time and/or frequency region and reference signal transmissions from user equipment not associated with the first access point use a second time and/or frequency region.
Preferably the method further comprises sending, from the first access point to a second access point, reference signal power information and a cyclic shift for the reference signal.
Preferably the method further comprises: determining whether to cancel the scheduling assignment; and in response to determining to cancel the scheduling assignment, informing the first user equipment that the scheduling assignment has been canceled.
Preferably the reference signal comprises a sounding reference signal.
Alternatively the reference signal comprises a physical random access channel signal.
In a second aspect there is provided a computer program product comprising computer executable instructions which when run on one or more processors perform: sending a scheduling assignment to a first user equipment at a first time; receiving a reference signal transmission from the first user equipment at a second time; and sending a downlink data packet transmission to the first user equipment at a third time.
Preferably the reference signal transmission from the first user equipment is a first reference signal transmission, and the actions further comprise: estimating, at the second time, reference signal resources, over which a second reference signal transmission from a second user equipment associated with a second access point is received, where the second reference signal transmission is associated with a second downlink data packet transmission from a second access point at the third time; and choosing a transmission strategy for use at the third time based at least in part on the estimated reference signal resources.
Preferably the downlink data packet transmission at the third time is a coordinated multipoint transmission.
Preferably, where, in coordinated beamforming coordinated multipoint transmission, the received reference signal transmission is used to one of:
design a downlink transmit weight and choose a precoder from a codebook.
Preferably, where, in dynamic access point selection coordinated multipoint transmission, the reference signal transmission is used in coordination with reference signal reception at other access points in a coordinated multipoint transmission set to choose a transmission access point to serve at least one other user equipment at the third time.
Preferably, where, in dynamic blanking coordinated multipoint transmission, the reference signal transmission is used in coordination with reference signal reception at other access points in a coordinated multipoint transmission set to determine whether the second access point should transmit to at least one other user equipment at the third time, where the coordinated multipoint transmission set comprises the second access point.
Preferably the actions further comprise designing beamforming weights based on the second reference signal transmission.
Preferably the beamforming weights generate spatial nulling in a direction towards the first user equipment.
Preferably a bandwidth of the reference signal transmission is equal to a bandwidth of a downlink radio barrier.
Preferably the receiving comprises separating the reference signal transmission using a transmission comb.
Preferably reference signal transmissions from user equipment associated with the first access point use a first finger in the transmission comb and reference signal transmissions from user equipment not associated with the first access point use a second finger in the transmission comb.
Preferably reference signal transmissions from user equipment associated with the first access point use a first time and/or frequency region and reference signal transmissions from user equipment not associated with the first access point use a second time and/or frequency region.
Preferably the actions further comprise sending, to a second access point, reference signal power information and a cyclic shift for the reference signal.
Preferably the actions further comprise: determining whether to cancel the scheduling assignment; and in response to determining to cancel the scheduling assignment, informing the first user equipment that the scheduling assignment has been canceled.
Preferably the computer program product is embodied on a computer readable medium.
Preferably the reference signal comprises a sounding reference signal.
Alternatively the reference signal comprises a physical random access channel signal.
In a third aspect there is provided an apparatus comprising at least one processor; and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following: to send a scheduling assignment to a first user equipment at a first time; to receive a first reference signal transmission from the first user equipment at a second time; and to send a first downlink data packet transmission to the first user equipment at a third time.
Preferably the at least one memory and the computer program code are further configured to cause the apparatus:
to estimate, at the second time, reference signal resources, over which a second reference signal transmission from a second user equipment associated with a second access point is received, where the second reference signal transmission is associated with a second downlink data packet transmission from a second apparatus at the third time; and to choose a transmission strategy for use at the third time based at least in part on the estimated reference signal resources.
Preferably the second downlink data packet transmission at the third time is a coordinated multipoint transmission.
Preferably in coordinated beamforming coordinated multipoint transmission, the second reference signal transmission is used to one of: design a downlink transmit weight and choose a precoder from a codebook.
Preferably the at least one memory and the computer program code are further configured to cause the apparatus to design beamforming weights based on the second reference signal transmission.
Preferably the beamforming weights generate spatial nulling in a direction towards the second user equipment.
Preferably a bandwidth of the first reference signal transmission is equal to a bandwidth of a downlink radio barrier.
Preferably, when receiving the reference signal transmission, the at least one memory and the computer program code are further configured to cause the apparatus to separate the first reference signal transmission using a transmission comb.
Preferably reference signal transmissions from user equipment associated with the first access point use a first finger in the transmission comb and reference signal transmissions from user equipment not associated with the first access point use a second finger in the transmission comb.
Preferably reference signal transmissions from user equipment associated with the first access point use a first time and/or frequency region and reference signal transmissions from user equipment not associated with the first access point use a second time and/or frequency region.
Preferably the at least one memory and the computer program code are further configured to cause the apparatus to send, to a second access point, reference signal power information and a cyclic shift for the first reference signal.
Preferably the at least one memory and the computer program code are further configured to cause the apparatus: to determine whether to cancel the scheduling assignment; and in response to determining to cancel the scheduling assignment, to inform the first user equipment that the scheduling assignment has been canceled.
Preferably the apparatus comprises an access point.
Alternatively the apparatus comprises a base station.
Preferably the reference signal comprises a sounding reference signal.
Alternatively the reference signal comprises a physical random access channel signal.
In a fourth aspect there is provided an apparatus comprising: means for sending a scheduling assignment to a first user equipment at a first time; means for receiving a reference signal transmission from the first user equipment at a second time; and means for sending a downlink data packet transmission to the first user equipment at a third time.
Preferably the reference signal transmission from the first user equipment is a first reference signal transmission, and the apparatus further comprises:
means for estimating, at the second time, reference signal resources, over which a second reference signal transmission from a second user equipment associated with a second access point is received, where the second reference signal transmission is associated with a second downlink data packet transmission from a second access point at the third time; and means for choosing a transmission strategy for use at the third time based at least in part on the estimated reference signal resources.
Preferably the downlink data packet transmission at the third time is a coordinated multipoint transmission.
Preferably, where, in coordinated beamforming coordinated multipoint transmission, the received reference signal transmission is used to one of: design a downlink transmit weight and choose a precoder from a codebook.
Preferably, where, in dynamic access point selection coordinated multipoint transmission, the reference signal transmission is used in coordination with reference signal reception at other access points in a coordinated multipoint transmission set to choose a transmission access point to serve at least one other user equipment at the third time.
Preferably, where, in dynamic blanking coordinated multipoint transmission, the reference signal transmission is used in coordination with reference signal reception at other access points in a coordinated multipoint transmission set to determine whether the second access point should transmit to at least one other user equipment at the third time, where the coordinated multipoint transmission set comprises the second access point.
Preferably the apparatus further comprises means for designing beamforming weights based on the second reference signal transmission.
Preferably the beamforming weights generate spatial nulling in a direction towards the first user equipment.
Preferably a bandwidth of the reference signal transmission is equal to a bandwidth of a downlink radio barrier.
Preferably the receiving means comprise means for separating the reference signal transmission using a transmission comb.
Preferably reference signal transmissions from user equipment associated with the first access point use a first finger in the transmission comb and reference signal transmissions from user equipment not associated with the first access point use a second finger in the transmission comb.
Preferably reference signal transmissions from user equipment associated with the first access point use a first time and/or frequency region and reference signal transmissions from user equipment not associated with the first access point use a second time and/or frequency region.
Preferably the apparatus further comprises means for sending, to a second access point, reference signal power information and a cyclic shift for the reference signal.
Preferably the apparatus further comprises: means for determining whether to cancel the scheduling assignment; and means for informing the first user equipment that the scheduling assignment has been canceled in response to determining to cancel the scheduling assignment.
Preferably the reference signal comprises a sounding reference signal.
Alternatively the reference signal comprises a physical random access channel signal.